If you fancy becoming a cardinal, you could always enrol in a theology course, enter a seminary, get ordained as a priest, work your way up to bishop, and then try to bring yourself to the notice – in a good way – of the pope. Being Italian wouldn't hurt.

If, however, you don't fancy all the hard work and prayer, you could just put on the frock and join in. This was the strategy of Ralph Napierski, an impostor who donned some vestments and mingled with the College of Cardinals, getting his photo taken and making it as far as the square in front of the Paul VI audience hall before being carted off.

What let him down, in the end, was his wardrobe. The Swiss Guard – the Vatican's official fashion police – noticed that the hem of his cassock was too high, that his red sash was actually just a purple scarf tied round his waist and that in place of the traditional red skull cap he was wearing a black fedora.

There are many reasons one might want to pretend to be something one is not: to gain money, esteem, preferment, privilege, or a stranger's misplaced confidence. But increasingly impostors attempt to pass themselves off in situations where the benefits are small and the prospect of getting caught almost certain. Ralph Napierski is a self-appointed bishop of a made-up church who claimed to be lodging a protest about child abuse, but his 15 seconds of fame didn't offer much of a platform.

Some people, clearly, do it for the sake of it. In the 90s a man named Jerry Whittredge posed as an astronaut to gain access to Nasa mission control. Karl Power from Manchester has made a name for himself with brief forays into sporting imposture: he got himself in the Manchester United team photo before a Champions League match in 2001, walked out to bat with the English cricket team at Headingley the same year, and later hit a few balls on Wimbledon's centre court before a Tim Henman match.

Michaele Salahi and her husband Tareq meet Barack Obama at the White House – without an invite. Photograph: Ho/REUTERS

Harder still to guess at are the motivations of Michaele and Tareq Salahi, who got all dressed up and talked their way into a White House banquet, bypassing tight security and risking criminal charges in the process. Their lawyer said they were cleared to enter the White House, which is not quite the same as being invited.

In the end we tend to admire impostors, if only because we have all been made to feel like one at some point. If you've ever been turned away from a place for not wearing a tie, or lied in order to breach a velvet rope – even just to use the loo – you can probably empathise. If you're making your own army medals out of papier-mache, you've gone too far.